Axon regeneration failure in the adult mammalian central nervous system (CMS) has been attributed at least in part to the inhibitory nature of the CNS myelin. Three "classical" myelin-derived neurite outgrowth inhibitors, Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (OMgp), have been identified that exhibit potent inhibitory activity on neurite outgrowth in vitro. However, the central question remains as to the contribution of these inhibitors to CNS axon regeneration failure in vivo. Our central hypothesis is that myelin-derived neurite growth inhibitors Nogo, MAG and OMgp play a significant and potentially redundant role in blocking spinal axon regeneration. The overall approach is to examine axon regeneration in the spinal cord of mice with gene deletion in one or more myelin inhibitors chronically or acutely, specifically addressing the issues of developmental compensation and functional redundancy. By acute gene deletion, we address the issue of developmental compensation in germline mutants. By deleting the three inhibitors simultaneously, we address the issue of functional redundancy. Aim 1. To assess the role of Nogo and OMgp in regeneration failure by examining the effect of acutely deleting Nogo or OMgp on corticospinal and raphespinal serotonergic fiber regeneration. We will employ an inducible knockout system to acutely delete Nogo or OMgp in oligodendrocytes and then examine the regenerative response of the corticospinal tract (CST) and the raphespinal serotonergic fiber tract. Aim 2. To assess the combined contribution of Nogo, MAG and OMgp in CNS axon regeneration failure by ascertaining the regeneration potential of the corticospinal and raphespinal serotonergic fiber tracts in mice deficient in all three inhibitors. Aim 3. To test whether increasing the intrinsic growth potential has a synergistic effect with removing myelin inhibitors in promoting spinal axon regeneration. Previous studies indicate that a conditioning lesion to the peripheral branch of the dorsal root ganglion (DRG) neurons augments the intrinsic growth potential of the neurons and promotes the regeneration of the central branch in vivo. We will test whether this enhanced regeneration is further enhanced in Nogo/MAG/OMgp triple mutant, and whether there is a synergistic effect between deleting the three myelin inhibitors and a conditioning lesion. Together, these studies will provide important insight into the role of myelin inhibitors in spinal axon regeneration failure. Understanding the role of these myelin inhibitors in CNS axon regeneration is crucial to the design of any therapeutic intervention to promote axon regeneration and spinal cord repair targeting this group of molecules. [unreadable] [unreadable] [unreadable] [unreadable]